linux-kernel - Re: [PATCH v4 2/2] dma/imx-sdma: convert _raw_readl/_raw

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Date:	Mon, 9 Jan 2012 14:12:14 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Eric Miao <eric.miao@...aro.org>
Cc:	Richard Zhao <richard.zhao@...aro.org>, patches@...aro.org,
	vinod.koul@...el.com, linux-kernel@...r.kernel.org,
	kernel@...gutronix.de, dan.j.williams@...el.com,
	shawn.guo@...aro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 2/2] dma/imx-sdma: convert _raw_readl/_raw_writel to
	readl/writel

On Mon, Jan 09, 2012 at 09:25:12PM +0800, Eric Miao wrote:
> Does this also mean when endian conversion is not necessary, the __raw_*
> version will be better here? Or generally the _relaxed variants are more
> recommended as endian conversion will be optimized away anyway with
> these AMBA accesses as both sides are little-endian?

Useless endian conversions are always optimized away.  Here's the
definitions:

If your CPU is operating in little endian mode, for 32-bit and 16-bit:

#define __cpu_to_le32(x) ((__force __le32)(__u32)(x))
#define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
#define __cpu_to_le16(x) ((__force __le16)(__u16)(x))
#define __le16_to_cpu(x) ((__force __u16)(__le16)(x))

So these are just casts to keep sparse happy and able to check this stuff.

#define __cpu_to_be32(x) ((__force __be32)__swab32((x)))
#define __be32_to_cpu(x) __swab32((__force __u32)(__be32)(x))
#define __cpu_to_be16(x) ((__force __be16)__swab16((x)))
#define __be16_to_cpu(x) __swab16((__force __u16)(__be16)(x))

These do the endian conversion.

If your CPU is running in big endian mode:

#define __cpu_to_le32(x) ((__force __le32)__swab32((x)))
#define __le32_to_cpu(x) __swab32((__force __u32)(__le32)(x))
#define __cpu_to_le16(x) ((__force __le16)__swab16((x)))
#define __le16_to_cpu(x) __swab16((__force __u16)(__le16)(x))

So these do the endian conversion, and:

#define __cpu_to_be32(x) ((__force __be32)(__u32)(x))
#define __be32_to_cpu(x) ((__force __u32)(__be32)(x))
#define __cpu_to_be16(x) ((__force __be16)(__u16)(x))
#define __be16_to_cpu(x) ((__force __u16)(__be16)(x))

These are just casts.
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